Multi-junction photovoltaic cells

ABSTRACT

A photovoltaic device including a photovoltaic cell and method of use is disclosed. The photovoltaic cell includes at least a first photovoltaic layer and a second photovoltaic layer arranged in a stack. The first photovoltaic layer has a first thickness and receives light at its top surface. A second photovoltaic layer has a second thickness and is disposed beneath the first photovoltaic layer and receives light passing through the first photovoltaic layer. The first thickness and the second thickness are selected so that a first light absorption at the first photovoltaic layer is equal to a second light absorption at the second photovoltaic layer. The photovoltaic cell is irradiated at its top surface with monochromatic light to generate a current.

BACKGROUND

The present invention relates in general to photovoltaic devices andmore specifically, to a multi-junction photovoltaic cell for generatingelectricity from monochromatic light.

Photovoltaic (PV) technology involves the conversion of light intoelectricity using semiconducting materials that exhibit the photovoltaiceffect, which is the creation of voltage or electric current in amaterial upon exposure to light. PV cells are generally integrated ontocircuit boards in order to power electrical elements of the circuitboard. As power demands of the circuit board elements increase, there isa need to add photovoltaic cells to the circuit board, causing aconflict between space dedicated to photovoltaic cells and spacededicated to circuit board elements.

SUMMARY

Embodiments of the present invention are directed to a photovoltaicdevice including a photovoltaic cell, the photovoltaic cell having: afirst photovoltaic layer that receives light at a top surface of thefirst photovoltaic layer, the first photovoltaic layer having a firstthickness; and a second photovoltaic layer disposed beneath the firstphotovoltaic layer that receives light passing through the firstphotovoltaic layer, the second photovoltaic layer having a secondthickness; wherein the first thickness and the second thickness areselected so that a first light absorption at the first photovoltaiclayer is equal to a second light absorption at the second photovoltaiclayer.

Embodiments of the present invention are further directed to aphotovoltaic device including a photovoltaic cell having a plurality ofphotovoltaic layers arranged in a stack, wherein a thickness of each ofthe plurality of photovoltaic layer is selected so that an amount oflight absorption absorbed at each of the plurality of photovoltaic layeris the same.

Embodiments of the present invention are further directed to a method ofgenerating electrical energy, including: irradiating a top surface of aphotovoltaic cell with monochromatic light, the photovoltaic cellincluding a stack of photovoltaic layers that generate a current whenirradiated at a wavelength of the monochromatic light, wherein athickness of each of the photovoltaic layers is selected so that each ofthe photovoltaic layers absorbed a same amount of light.

Additional technical features and benefits are realized through thetechniques of the present invention. Embodiments and aspects of theinvention are described in detail herein and are considered a part ofthe claimed subject matter. For a better understanding, refer to thedetailed description and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The specifics of the exclusive rights described herein are particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features and advantages ofthe embodiments of the invention are apparent from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 depicts a perspective view of a semiconductor photovoltaic devicesuitable for operating on electricity generated in response to lightreceived at the photovoltaic device according to embodiments of thepresent invention;

FIG. 2 depicts a block diagram of a photovoltaic cell of thephotovoltaic device of FIG. 1 in various embodiment of the presentinvention;

FIG. 3 depicts a graph of an absorption coefficient vs. wavelength forInGaP;

FIG. 4 depicts a chart of calculated thicknesses for the photovoltaiclayers of photovoltaic cells irradiated with coherent light at awavelength of 660 nanometers;

FIG. 5 depicts a graph of current vs. output voltage for a photovoltaiclayer of a photovoltaic cell including a photovoltaic material made ofInGaP;

FIG. 6 depicts a block diagram of a multi-junction photovoltaic deviceusing three GaAs p-material, n-material (PN) diodes; and

FIG. 7 depicts a block diagram of a multi-junction photovoltaic deviceusing five GaAs PN diodes.

The diagrams depicted herein are illustrative. There can be manyvariations to the diagram or the operations described therein withoutdeparting from the spirit of the invention. For instance, the actionscan be performed in a differing order or actions can be added, deletedor modified. Also, the term “coupled” and variations thereof describeshaving a communications path between two elements and does not imply adirect connection between the elements with no interveningelements/connections between them. All of these variations areconsidered a part of the specification.

In the accompanying figures and following detailed description of thedisclosed embodiments, the various elements illustrated in the figuresare provided with two or three digit reference numbers. With minorexceptions, the leftmost digit(s) of each reference number correspond tothe figure in which its element is first illustrated.

DETAILED DESCRIPTION

Various embodiments of the invention are described herein with referenceto the related drawings. Alternative embodiments of the invention can bedevised without departing from the scope of this invention. Variousconnections and positional relationships (e.g., over, below, adjacent,etc.) are set forth between elements in the following description and inthe drawings. These connections and/or positional relationships, unlessspecified otherwise, can be direct or indirect, and the presentinvention is not intended to be limiting in this respect. Accordingly, acoupling of entities can refer to either a direct or an indirectcoupling, and a positional relationship between entities can be a director indirect positional relationship. Moreover, the various tasks andprocess steps described herein can be incorporated into a morecomprehensive procedure or process having additional steps orfunctionality not described in detail herein.

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

Additionally, the term “exemplary” is used herein to mean “serving as anexample, instance or illustration.” Any embodiment or design describedherein as “exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs. The terms “at least one”and “one or more” may be understood to include any integer numbergreater than or equal to one, i.e. one, two, three, four, etc. The terms“a plurality” may be understood to include any integer number greaterthan or equal to two, i.e. two, three, four, five, etc. The term“connection” may include both an indirect “connection” and a direct“connection.”

The terms “about,” “substantially,” “approximately,” and variationsthereof, are intended to include the degree of error associated withmeasurement of the particular quantity based upon the equipmentavailable at the time of filing the application. For example, “about”can include a range of ±8% or 5%, or 2% of a given value.

For the sake of brevity, conventional techniques related to making andusing aspects of the invention may or may not be described in detailherein. In particular, various aspects of computing systems and specificcomputer programs to implement the various technical features describedherein are well known. Accordingly, in the interest of brevity, manyconventional implementation details are only mentioned briefly herein orare omitted entirely without providing the well-known system and/orprocess details.

By way of background, however, a more general description of thesemiconductor device fabrication processes that can be utilized inimplementing one or more embodiments of the present invention will nowbe provided. Although specific fabrication operations used inimplementing one or more embodiments of the present invention can beindividually known, the described combination of operations and/orresulting structures of the present invention are unique. Thus, theunique combination of the operations described in connection with thefabrication of a semiconductor device according to the present inventionutilize a variety of individually known physical and chemical processesperformed on a semiconductor (e.g., silicon) substrate, some of whichare described in the immediately following paragraphs.

In general, the various processes used to form a micro-chip that will bepackaged into an IC fall into four general categories, namely, filmdeposition, removal/etching, semiconductor doping andpatterning/lithography. Deposition is any process that grows, coats, orotherwise transfers a material onto the wafer. Available technologiesinclude physical vapor deposition (PVD), chemical vapor deposition(CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE)and more recently, atomic layer deposition (ALD) among others.Removal/etching is any process that removes material from the wafer.Examples include etch processes (either wet or dry), andchemical-mechanical planarization (CMP), and the like. Semiconductordoping is the modification of electrical properties by doping, forexample, transistor sources and drains, generally by diffusion and/or byion implantation. These doping processes are followed by furnaceannealing or by rapid thermal annealing (RTA). Annealing serves toactivate the implanted dopants. Films of both conductors (e.g.,poly-silicon, aluminum, copper, etc.) and insulators (e.g., variousforms of silicon dioxide, silicon nitride, etc.) are used to connect andisolate transistors and their components. Selective doping of variousregions of the semiconductor substrate allows the conductivity of thesubstrate to be changed with the application of voltage. By creatingstructures of these various components, millions of transistors can bebuilt and wired together to form the complex circuitry of a modernmicroelectronic device. Semiconductor lithography is the formation ofthree-dimensional relief images or patterns on the semiconductorsubstrate for subsequent transfer of the pattern to the substrate. Insemiconductor lithography, the patterns are formed by a light sensitivepolymer called a photo-resist. To build the complex structures that makeup a transistor and the many wires that connect the millions oftransistors of a circuit, lithography and etch pattern transfer stepsare repeated multiple times. Each pattern being printed on the wafer isaligned to the previously formed patterns and slowly the conductors,insulators and selectively doped regions are built up to form the finaldevice.

Epitaxial materials can be grown from gaseous or liquid precursors.Epitaxial materials can be grown using vapor-phase epitaxy (VPE),molecular-beam epitaxy (MBE), liquid-phase epitaxy (LPE), or othersuitable process. Epitaxial silicon, silicon germanium, and/or carbondoped silicon (Si:C) can be doped during deposition (in-situ doped) byadding dopants, n-type dopants (e.g., phosphorus or arsenic) or p-typedopants (e.g., boron or gallium), depending on the type of transistor.The dopant concentration in the source/drain can range from 1×10¹⁹ cm⁻³to 2×10²¹ cm⁻³, or preferably between 2×10²⁰ cm⁻³ and 1×10²¹ cm⁻³.

The terms “epitaxial growth and/or deposition” and “epitaxially formedand/or grown” mean the growth of a semiconductor material (crystallinematerial) on a deposition surface of another semiconductor material(crystalline material), in which the semiconductor material being grown(crystalline overlayer) has substantially the same crystallinecharacteristics as the semiconductor material of the deposition surface(seed material). In an epitaxial deposition process, the chemicalreactants provided by the source gases are controlled and the systemparameters are set so that the depositing atoms arrive at the depositionsurface of the semiconductor substrate with sufficient energy to moveabout on the surface such that the depositing atoms orient themselves tothe crystal arrangement of the atoms of the deposition surface.Therefore, an epitaxially grown semiconductor material has substantiallythe same crystalline characteristics as the deposition surface on whichthe epitaxially grown material is formed. For example, an epitaxiallygrown semiconductor material deposited on a {100} orientated crystallinesurface will take on a {100} orientation. In some embodiments, epitaxialgrowth and/or deposition processes are selective to forming onsemiconductor surface, and generally do not deposit material on exposedsurfaces, such as silicon dioxide or silicon nitride surfaces.

In some embodiments, the gas source for the deposition of epitaxialsemiconductor material include a silicon containing gas source, agermanium containing gas source, or a combination thereof. For example,an epitaxial Si layer can be deposited from a silicon gas source that isselected from the group consisting of silane, disilane, trisilane,tetrasilane, hexachlorodisilane, tetrachlorosilane, dichlorosilane,trichlorosilane, methylsilane, dimethylsilane, ethylsilane,methyldisilane, dimethyldisilane, hexamethyldisilane and combinationsthereof. An epitaxial germanium layer can be deposited from a germaniumgas source that is selected from the group consisting of germane,digermane, halogermane, dichlorogermane, trichlorogermane,tetrachlorogermane and combinations thereof. While an epitaxial silicongermanium alloy layer can be formed utilizing a combination of such gassources. Carrier gases like hydrogen, nitrogen, helium and argon can beused.

Turning now to an overview of technologies that are more specificallyrelevant to aspects of the invention, conventional photovoltaic (PV)cells possess an efficiency rating of approximately 10-30% when exposedto natural or white light, thereby allowing the use of conventional PVcells to provide a selected amount of power. However, circuit elementsthat require power can include processors, memory cells, power storagedevices, etc., which are built with increasing power demands. Asintegrated circuit elements that could potentially draw power from thephotovoltaic power source become more complex, there is a need toprovide PV technology that can generate higher voltage. However, togenerate higher voltages, multiple PV cells are often formed on thesubstrate, where they are joined electrically in a series connection.Multiple PV cells, however, have the negative consequence of consuminggreater amounts of space on the integrated circuit substrate.

Turning now to an overview of the aspects of the invention, one or moreembodiments of the invention address the above-described shortcomings ofthe prior art by providing provide a photovoltaic cell capable of highpower output without additional circuit board space requirements. Thephotovoltaic cell includes photovoltaic layers (PV layers) arranged in astack in order to reduce a footprint of the PV cell, thus minimizing anamount of space or real estate used on a substrate. The PV layers havehigh efficiencies (e.g., up to about 60%) of optical-electricalconversion when exposed to coherent light (e.g., laser light). Byarranging the PV layers in a stack and by varying the thicknesses ofeach PV layer, a high-efficiency (about 40-60%) multi-junctionphotovoltaic (MJ-PV) device can be formed. The thicknesses of the PVlayers are selected so that each PV layer absorbs the same amount oflight for the wavelength of the incident light.

The above-described aspects of the invention address the shortcomings ofthe prior art by increasing the thickness of each PV layer with depth,whereas depth is measured from the surface of the PV cell onto which thelight is incident. The thickness of each PV layer can be determinedbased at least in part on an amount of light absorption occurring ateach PV layer.

Turning now to a more detailed description of aspects of the presentinvention, FIG. 1 depicts a perspective view of a semiconductorphotovoltaic device 100 suitable for operating on electricity generatedin response to light received at the photovoltaic device 100 accordingto embodiments of the present invention. In various embodiments, thephotovoltaic device 100 may be a processor assembly that operates onoptical power. The device 100 includes a memory layer 102 having amemory device formed therein, a processor layer 104 having a processorformed therein, and a PV cell 106 for converting light into electricalcurrent and to provide a voltage for powering the device 100. The device100 further includes an LED 108 formed on the processor layer 104 andcontrolled by the processor to transmit optical signals.

In various embodiments of the invention, the memory layer 102 is abottom layer of the device 100 and the processor layer 104 is formed ontop of the memory layer 102. Wired connections between the processorlayer 104 and the memory layer 102 provide data transfer between theprocessor of the processor layer 104 and the memory device of the memorylayer 102. Therefore, the processor can store data to the memory deviceand retrieve data from the memory device.

PV cell 106 is formed on top of the processor layer 104 and is in wiredconnection with the processor layer 104 to provide power to theprocessor layer 104. Light 112 from external light source 110 isincident on a top surface of the PV cell 106. The external light source110 provides coherent monochromatic light 112 to the PV cell 106 and thematerial of the PV cell 106 is selected to generate electricity whenirradiated at the wavelength of the incident light 112. In variousembodiments of the invention, the specific wavelength of light is about660 nanometers. The external light source 110 may be a laser lightsource, an LED or any other monochromatic light source.

In various embodiments of the invention, photovoltaic cell 106 generatesfrom about 3.0 volts to about 4.0 volts to power the PV device 100. Theelectricity generated by the photovoltaic cell 106 is used to provideelectrical power to the processor layer 104 and memory layer 102 and toprovide power for operating the LED 108. In various embodiments of theinvention, a driving voltage of the LED 108 is from about 2 volts toabout 3 volts. In various embodiments of the invention, the photovoltaiccell 106 can charge a battery (not shown) to a voltage of about 3.5volts. In various embodiments of the invention, a foot print of thephotovoltaic device 100 can be about 100 micrometers by 100 micrometers.

FIG. 2 depicts a block diagram of the PV cell 106 in various embodimentsof the invention of the present invention. The PV cell 106 includes aplurality of photovoltaic layers arranged in a stack. A tunnel layer issandwiched between each of the plurality of photovoltaic layers. Anupper surface of the tunnel layer is in direct contact with thephotovoltaic layer above the tunnel layer, and a lower surface of thetunnel layer is in direct contact with the photovoltaic layer below thetunnel layer. In the illustrative embodiment of FIG. 2, the photovoltaiccell 106 includes three photovoltaic layers. However, the number ofphotovoltaic layer is not meant to be a limitation of the invention. Thephotovoltaic cell 106 can include any number of photovoltaic layers tomeet various technical specifications.

Illustrative photovoltaic cell 106 includes first PV layer 202, secondPV layer 206, and third PV layer 210, with the first PV layer 202 beingat the top of the PV cell 106, the second PV layer 206 being in themiddle of the PV cell 106 and the third PV layer 210 being at the bottomof the PV cell 106. A first tunnel layer 204 is sandwiched between thefirst PV layer 202 and the second PV layer 206. A second tunnel layer208 is sandwiched between second PV layer 206 and third PV layer 210.The first PV layer 202, second PV layer 206 and third PV layer 210 arelinked in series as shown by electrical connections 220 and 222. Byconnecting the PV layers 202, 206 210, the total voltage output of thePV cell 106 is a summation of the voltage outputs at each PV layer 202,206, 210. Additionally, the voltage can be increased by adding anotherPV layer to the stack of PV layers, thereby allowing the PV cell to becreated that provides a particular voltage output. Light 112 is incidentfrom the external light source 110 at a top surface of the first PVlayer 202 and passes through first tunnel layer 204, second PV layer206, second tunnel layer 208 and third PV layer 210.

The intensity of light diminishes as the light passes through each PVlayer (202, 206, 210). The thickness of a selected PV layer and theabsorption coefficient of the material of the PV layer determines theamount of light absorbed by the selected PV layer and therefore theamount of electricity and/or voltage generated by the selected PV layer.The thickness of the PV layers 202, 206 and 210 are selected so that thefirst PV layer 202, the second PV layer 206, and the third PV layer 210generate the same or approximately the same open-circuit voltage(V_(OC)) when subject to the light provided to the particular layer. Theintensity of light provided to the top surface of the second PV layer206 is less than the intensity of light provided to the top surface ofthe first PV layer 202 due to light absorption at the first PV layer202. Therefore, the thickness of the second PV layer 206 is greater thanthe thickness of the first PV layer 202 so that both PV layers 202 and206 produce the same V_(OC). Similarly, the intensity of light incidentat the top surface of the third PV layer 210 is less than the intensityof light incident at the top surface of the second PV layer 206 due tolight absorption at the second PV layer 206. Therefore, the thickness ofthe third PV layer 210 is greater than the thickness of the second PVlayer 206 so that both PV layers 206 and 210 produce the same V_(OC).

In order to determine the thickness the PV layers 202, 206, 210, lightabsorption calculations are made to provide that the amount of lightabsorbed at each PV layer is the same for each PV layer. The intensityof light incident at the top surface of the first PV layer 202 isrepresented as I₀. Similarly, the intensity of light exiting the firstPV layer 202 and incident on the second PV layer 206 is represented asI₁, and the intensity of light exiting the second PV layer 206 andincident on the third PV layer 210 is represented as I₂. Finally, theintensity of light exiting the bottom surface of the third PV layer 210is represented as I₃. The relation between these intensities is detailedin Eqs. (1)-(3) below:

I ₁ =I ₀exp(−αd ₁)  Eq. (1)

I ₂ =I ₁exp(−αd ₂)  Eq. (2)

I ₃ =I ₂exp(−αd ₃)  Eq. (3)

where α is the absorption coefficient of the material of the PV layers,d₁ is the thickness of the first PV layer 202, d₂ is the thickness ofthe second PV layer 206 and d₃ is the thickness of the third PV layer210. Because the PV layers are made of the same material, the absorptioncoefficient α of each PV layer is identical. Eq. (4) represents acondition of equal light absorption at each PV layer:

I ₀ −I ₁ =I ₁ −I ₂ =I ₂ −I ₃  Eq. (4)

From Eqs. (1)-(4), it can be determined that

1−exp(−αd ₁)=exp(−αd ₁)(1−exp(−αd ₂))=exp(−αd ₁)exp(−αd ₂)(1−exp(−αd₃))  Eq. (5)

The first equality of Eq. (5) can be used to determine a relationbetween d₁ and d₂. These values can then be used in the second equalityof Eq. (5) to determine the relation between d₃ and d₁, d₂. For a PVcell 106 having additional PV layers, Eqs. (4) and (5) can be extendedappropriately to determine the relation between the thicknesses of allof the PV layers.

In various embodiments of the invention, the thicknesses of the PVlayers are selected so that the intensity of light exiting thebottom-most PV layers (I₃ in the illustrative embodiment of FIG. 2) issubstantially zero. Therefore, for a PV cell having N PV layers, each PVlayer absorbs 1/N of the incident light.

The thickness of the PV layers is dependent on the extinctioncoefficient or absorption coefficient of the material. FIG. 3 depicts agraph 300 of an absorption coefficient α vs. wavelength for InGaP.Values of wavelength of light shown on the x-axis 302 and values of theextinction coefficient α are shown on the y-axis 304. Values of α areselected for the desired wavelength of the incident light, and selectedvalues are used to determine layer thicknesses.

FIG. 4 depicts a chart 400 of calculated layer thicknesses for the PVlayers of PV cells irradiated with coherent light at a wavelength of 660nm. The material of the PV layers is indium gallium phosphide (InGaP).The thickness is dependent upon the PV diode's location within the PVdevice 100. The chart 400 includes column 402 disclosing layerthicknesses for a PV cell having two PV layers, column 404 disclosinglayer thicknesses for a PV cell having three PV layers, column 406disclosing layer thicknesses for a PV cell having four PV layers, andcolumn 408 disclosing layer thicknesses for a PV cell having five PVlayers. Layer thicknesses were determined using Equations 1-5, adjustedfor the particular number of PV layers. The chart 400 shows that thethickness of each layer increases with the depth of the layer. Theincreased PV layer thickness with depth compensates for attenuation oflight intensity with depth so that each PV layer generates a sameV_(OC).

FIG. 5 depicts a graph 500 of current vs. output voltage for a layer ofa PV cell including a photovoltaic material made of InGaP. Voltage isshown along the x-axis 502 and current is shown along y-axis 504. Theopen circuit voltage V_(OC) is indicated as the voltage where thecurrent curve 508 drops to zero and is indicated at the voltageindicated at current extinction voltage 510. The shape of graph 500 andthe open circuit voltage V_(OC) is a property of the material of the PVcell. The V_(OC) of graph 500 is approximately 1.4 volts.

FIG. 6 depicts a block diagram of a MJ-PV device 600 using three GaAsp-material, n-material (PN) diodes in an embodiment of the invention.The MJ-PV device 600 includes a substrate 602, a first contact layer 604disposed onto the substrate 602 and comprising p+ GaAs materialapproximately 0.5 μm thick. A third PV layer subassembly 644 is disposedonto the first contact layer 604 and includes a BSF 606 comprised ofInGaP or AlGaAs, a p-type layer 608 comprised of a GaAs material andhaving a thickness of “d3”, an n-type GaAs emitter 610 having athickness of approximately 100 nm, and a window layer 612 comprising ann-type InGaP or AlGaAs material. The p-type layer 608 and the n-typeGaAs emitter 610 combine to form a PN diode 635. The first contact layer604 establishes an electrical connection to the third PV layersubassembly 640 and allows other devices and structures on the substrate602 to make a first electrical connection to the MJ-PV device 600.

Additional PV layer subassemblies are included in the MJ-PV device 600using substantially the same materials and layers as the first PV layersubassembly 644, but employing differing thicknesses of the PV diode 635depending on the location of the subassembly in the MJ-PV device 600.Additional subassemblies include a second PV layer subassembly 642 witha PN diode 623 having a thickness of “d2”, and a first PV layersubassembly 644 with a third PN diode 635 and having a thickness of“d1”.Thicknesses d1, d2, and d3 are determined by applying Equations 1-5 asdisclosed herein.

Continuing with the description of MJ-PV device 600, a second tunnellayer 646B is interposed between the third PV layer subassembly 644 andthe second PV layer subassembly 642. The second tunnel layer 646Bincludes two layers: an N+ Al_(0.3)Ga_(0.7)As layer 614 that isapproximately 20 nm thick and a P+ GaAs layer 616 that is approximately20 nm thick. The second tunnel layer 646B provides a serial electricalinterconnection between the third PV layer subassembly 644 and thesecond PV layer subassembly 642, therefore enabling current to flowbetween the third PV layer subassembly 644 and the second PV layersubassembly 642. Similarly, a first tunnel layer 646A is interposedbetween the second PV layer assembly 642 and the first PV layersubassembly 640 and provides a serial electrical interconnection betweenthe second PV layer subassembly 642 and the first PV layer subassembly640, therefore enabling current to flow between the second PV layersubassembly 642 and the first PV layer subassembly 640. The first tunnellayer 646A includes two layers: an N+ Al_(0.3)Ga_(0.7)As layer 624 thatis approximately 20 nm thick and a P+ GaAs layer 626 that isapproximately 20 nm thick. Also included in the MJ-PV device 600 is asecond contact layer 638 disposed onto the first PV layer subassembly644 and comprising n+ GaAs material approximately 0.5 μm thick. Thesecond contact layer 638 establishes an electrical connection to thefirst PV layer subassembly 644 and allows other devices and structureson the substrate 602 to establish a second electrical connection to theMJ-PV device 600.

FIG. 6, as described herein, includes first PN diode 611, second PNdiode 623, and third PN diode 635, where each diode has a V_(OC) ofapproximately 0.6 to 0.8 volts. As first PN diode 611, second PN diode623, and third PN diode 635 are connected serially, the total V_(OC) ofthe MJ-PV device 600 is approximately 1.8-2.4 volts. In at least someembodiments of the present invention, the V_(OC) is approximately 2.0volts. In this example, thicknesses for the first PN diode 611, secondPN diode 623, and third PN diode 635 are 260 nm, 480 nm, and 1500 nm,respectfully.

FIG. 7 depicts a block diagram of a MJ-PV device 700 using five GaAs PNdiodes in an embodiment of the invention. FIG. 7 uses substantially thesame components and devices as illustrated in FIG. 6 and accordingly,the same reference numbers as appropriate. Additional elements to theMJ-PV device 700 include a fourth PV layer subassembly 750 having afourth PN diode 752 and a fifth PV junction subassembly 755 having afifth PN diode 757. The MUJ-PV device also includes a third tunnel layer646C and a fourth tunnel layer 646D. In this arrangement, MJ-PV device700 generates a V_(OC) of approximately 3.5 volts employing GaAsmaterials. Thicknesses of the materials vary from those shown in FIG. 6.Thicknesses are approximately 100 nm (first PN diode 611), approximately150 nm (second PN diode 623), approximately 240 nm (third PN diode 735),approximately 420 nm (fourth PN diode 752), and approximately 1100 nm(fifth PN diode 757).

In some embodiments of the invention, various functions or acts can takeplace at a given location and/or in connection with the operation of oneor more apparatuses or systems. In some embodiments of the invention, aportion of a given function or act can be performed at a first device orlocation, and the remainder of the function or act can be performed atone or more additional devices or locations.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed.Embodiments of the present invention have been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the form described. Many modifications and variations will beapparent to those of ordinary skill in the art without departing fromthe scope and spirit of the invention. The embodiments of the inventionwere chosen and described in order to best explain the principles of theinvention and the practical application, and to enable others ofordinary skill in the art to understand the invention for variousembodiments of the invention with various modifications as are suited tothe particular use contemplated.

The flowchart and block diagrams in the figures illustrate thefunctionality and operation of possible implementations of systems andmethods according to various embodiments of the present invention. Insome alternative implementations, the functions noted in the block canoccur out of the order noted in the figures. For example, two blocksshown in succession can, in fact, be executed substantiallyconcurrently, or the blocks can sometimes be executed in the reverseorder, depending upon the functionality involved. The actions can beperformed in a differing order or actions can be added, deleted ormodified. Also, the term “coupled” describes having a signal pathbetween two elements and does not imply a direct connection between theelements with no intervening elements/connections therebetween. All ofthese variations are considered a part of the invention.

The terms “about,” “substantially,” “approximately,” and variationsthereof, are intended to include the degree of error associated withmeasurement of the particular quantity based upon the equipmentavailable at the time of filing the application. For example, “about”can include a range of ±8% or 5%, or 2% of a given value.

The terminology used herein is for describing particular embodiments ofthe invention only and is not intended to be limiting of embodiments ofthe present invention. As used herein, the singular forms “a”, “an” and“the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. It will be further understood thatthe terms “comprises” and/or “comprising,” when used in thisspecification, specify the presence of stated integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other integers, steps, operations,element components, and/or groups thereof.

While embodiments of the present invention have been described in detailin connection with only a limited number of embodiments, it should bereadily understood that embodiments of the present invention are notlimited to such described embodiments. Rather, embodiments of thepresent invention can be modified to incorporate any number ofvariations, alterations, substitutions or equivalent arrangements notheretofore described, but which are commensurate with the spirit andscope of the present invention. Additionally, while various embodimentsof the present invention have been described, it is to be understoodthat aspects of the present invention can include only some of thedescribed embodiments. Accordingly, the present invention is not to beseen as limited by the foregoing description, but is only limited by thescope of the appended claims.

1. A photovoltaic device, comprising: a photovoltaic cell, including: afirst photovoltaic layer that receives light at a top surface of thefirst photovoltaic layer, the first photovoltaic layer having a firstthickness; and a second photovoltaic layer disposed beneath the firstphotovoltaic layer that receives light passing through the firstphotovoltaic layer, the second photovoltaic layer having a secondthickness; wherein the first thickness and the second thickness areselected so that a first light absorption at the first photovoltaiclayer is equal to a second light absorption at the second photovoltaiclayer.
 2. The photovoltaic device of claim 1, further comprising atunnel layer sandwiched between the first photovoltaic layer and thesecond photovoltaic layer.
 3. The photovoltaic device of claim 1,wherein the light is monochromatic and a material of the first andsecond photovoltaic layers generates a current when irradiated at awavelength of the monochromatic light.
 4. The photovoltaic device ofclaim 1, wherein the first photovoltaic layer is electrically connectedin series with the second photovoltaic layer.
 5. The photovoltaic deviceof claim 1, further comprising a processor layer coupled to thephotovoltaic cell, wherein the processor layer operates using a currentgenerated by the photovoltaic cell.
 6. The photovoltaic device of claim5, further comprising a memory layer coupled to the processor layer. 7.The photovoltaic device of claim 5, further comprising a light emittingdevice (LED) coupled to the processor layer, wherein the processorcontrols operation of the LED and the LED operates using a currentgenerated by the photovoltaic cell.
 8. A photovoltaic device,comprising: a photovoltaic cell including a plurality of photovoltaiclayers arranged in a stack, wherein a thickness of each of the pluralityof photovoltaic layer is selected so that an amount of light absorptionabsorbed at each of the plurality of photovoltaic layer is the same. 9.The photovoltaic device of claim 8, further comprising a tunnel layersandwiched between each of the plurality of photovoltaic layer.
 10. Thephotovoltaic device of claim 8, wherein a voltage across each of theplurality of photovoltaic layers when irradiated by a monochromaticlight is substantially the same.
 11. The photovoltaic device of claim 8,wherein the light intensity at a bottom surface of the photovoltaic cellis substantially zero.
 12. The photovoltaic device of claim 8, furthercomprising a processor layer coupled to the photovoltaic cell, whereinthe processor layer operates using a current generated at thephotovoltaic cell.
 13. The photovoltaic device of claim 12, furthercomprising a memory layer coupled to the processor layer.
 14. Thephotovoltaic device of claim 12, further comprising a light emittingdevice (LED) coupled to the processor layer, wherein the processorcontrols operation of the LED layer to generate light and the LEDoperates using a current generated by the photovoltaic cell. 15.(canceled)
 16. (canceled)
 17. (canceled)
 18. (canceled)
 19. (canceled)20. (canceled)